Your premise is incorrect. Most use a single starting battery (sometimes relabeled as a combination battery but really just a starting battery if you open it up and look at the construction).
If you don't camp off grid, it's a perfectly good solution. Jacks & slides will generally pull less than 100amps for less than 30seconds and a starting battery does just fine doing that.
Only reason to consider Gel or AMG is if you are going to roll your RV. The big advantage to them is they don't leak out the acid if you put them on their side or upside down (planes, boats, jeeps can gain advantage as they often spend time substantially off level). They offer no real advantage in an RV.